As one of systems obtaining electric energy, a light-electricity conversion system employing sunlight has been studied.
As the systems employing sunlight, there are mentioned a sunlight power system generation system directly converting sunlight to electricity, and a solar thermal power generation (light collection type solar thermal power generation) system converting the heat of sunlight to electricity.
Of these, the solar thermal power generation has advantages in that it is less in environmental load, enables 24 hours' power generation due to stored heat, and is relatively high in energy conversion efficiency.
The solar thermal power generation is one in which sunlight is collected through a reflector such as a mirror and heat of the collected light is transferred to a thermal medium through which power is generated.
In the power generation, a thermal medium, for example, water, oil, a melted salt or the like is employed and heated and water is evaporated by the heated thermal medium, whereby a stream turbine is revolved to generate power.
The solar thermal power generation system has shortcomings in that a long sunshine duration and a vast tract of land are necessary and big equipments, which function with a stable performance even under severe weather conditions, are required, as compared with the sunlight power generation system.
As one of the big equipments, which function with a stable performance under severe weather conditions, there is a reflective mirror used for collecting sunlight.
As the reflective mirrors, there are generally known a glass mirror in which a reflector is provided on a glass plate and a film mirror in which a reflector is provided on a film other than glass, for example, a plastic film. They are properly employed depending on the usage.
As the film mirror, there is known a film mirror as disclosed in, for example, Japanese Patent O.P.I. Publication No. 2004-77643 which comprises a base film, a silver deposition film and a film for preventing corrosion provided on the base film in that order, and a back coat film provided on the surface of the base film opposite the anchor film.
As the reflective mirror for collecting sunlight, there is known a mirror as disclosed in Japanese Patent O.P.I. Publication No. 2005-106432 in which a synthetic resin such as FRP is plated to obtain a mirror surface or a mirror as disclosed in Japanese Patent O.P.I. Publication No. 2008-249166 in which a heat resistant resin such as a phenol resin, a fluorine-containing resin or a polyimide resin is vapor deposited or plated with aluminum to obtain a mirror surface.
A concave mirror as described later is known as an optical member (concave mirror), which is small in temperature variation of optical property, light in weight, and excellent in mass productivity.
A concave mirror is known which comprises a metal film with a reflective surface provided on the smooth surface of a base material prepared from a nano-composite as a raw material in which a microfibrillated cellulose assembly is impregnated with a plastic resin as a raw material (refer to Patent Document Nos. 1 and 2).
However, when these reflective mirrors are employed as a light collecting reflective mirror used for solar thermal power generation, light collecting performance greatly deteriorates under temperature or humidity variation in a place to be installed. Therefore, a reflective mirror having further stable light collecting ability has been required.
Specifically, when light is collected employing a heliostat, a reflective mirror with high specular reflectance has been desired, and further, a reflective mirror, which exhibits a high stable light collecting ability and particularly high stability of specular resistance under temperature or humidity variation in a place to be installed, has been desired.